Glossary term
Isentropic Efficiency
A thermodynamic performance metric comparing actual device work or enthalpy change with an ideal isentropic process.
Definition
metricIsentropic efficiency compares the actual performance of a turbine, compressor, pump, or nozzle with the ideal performance for an isentropic process between the same inlet and outlet pressure conditions.
An isentropic process is internally reversible and adiabatic, so entropy remains constant. Real turbomachinery and flow devices have friction, turbulence, heat transfer, leakage, shock losses, mixing, blade losses, and mechanical losses. Isentropic efficiency expresses how close the real enthalpy change or work transfer is to that ideal limit. The formula differs for turbines and compressors because turbines produce work while compressors and pumps consume work.
Isentropic efficiency measures how closely a real device approaches an ideal adiabatic and internally reversible process. It is used for turbines, compressors, pumps, nozzles, fans, and expanders. The ideal reference process has the same inlet state and the same outlet pressure as the real device, but entropy remains constant.
For a turbine, the ideal isentropic process gives the maximum possible work output for the pressure drop. The efficiency is commonly written:
where h_1 is inlet enthalpy, h_2 is actual outlet enthalpy, and h_{2s} is the outlet enthalpy for the ideal isentropic expansion to the same outlet pressure.
For a compressor or pump, the ideal isentropic process gives the minimum possible work input for the pressure rise:
The numerator and denominator reverse because compressors consume work.
Physical meaning
A lower isentropic efficiency means more of the available work potential is destroyed by irreversibilities. These include viscous friction, turbulence, leakage, heat transfer, blade tip clearance, shock waves, incidence losses, secondary flows, mechanical losses, and nonuniform inlet conditions. In exergy terms, the lost work is connected to entropy generation.
Isentropic efficiency is a component metric, not the same as overall thermal efficiency. A gas turbine can have high compressor and turbine isentropic efficiencies while the full cycle efficiency still depends on pressure ratio, turbine inlet temperature, combustor losses, recuperation, cooling flows, and exhaust heat recovery.
Use in engineering
Engineers use isentropic efficiency to size compressors and turbines, compare machinery, build cycle models, estimate power demand, diagnose degradation, and validate test data. In performance maps, efficiency varies with flow rate, pressure ratio, rotational speed, Reynolds number, inlet guide vane position, and operating point. A single efficiency value is meaningful only when the state point and definition are clear.
Common mistakes
A common mistake is using the turbine formula for a compressor or the compressor formula for a turbine. Another is mixing total and static properties inconsistently. For high-speed flows, turbomachinery and nozzles may require total enthalpy and total pressure definitions. Good documentation states whether the efficiency is total-to-total, total-to-static, polytropic or isentropic, dry or wet, corrected or uncorrected, and how inlet and outlet states were measured.